1. Field of the Invention
The present invention relates to a color image data processing method for reproducing a color image by use of micro color points of a plurality of colors.
2. Description of the Prior Art
In conventional printing, micro color points of each color of, e.g., cyan C, magenta M, yellow Y, and black K are used and by making the areas of color points different, various colors are reproduced.
The area of the color point of each color has been conventionally determined by performing processings such as masking processing, undercolor removal and inking in consideration of unnecessary absorption of respective colors of Y, M, C, and K which are actually used for the density at which the printing should be done on the basis of photograph technology. However, it is conventionally well known that this method is imperfect since the densities which are used have various problems. It is an old method in printing that the chrominance calculations are carried out with respect to eight colors of Y, M, C, K, red R, green G, blue B, and white W. An example of these chrominance calculations is disclosed in H. E. Neugebauer, "Z. Photo. 34 (4)", pages 73-89, 1937; A. C. Hardy et al, "J. Opt. S Am 38", pages 300-307, 1948, and the like.
However, in these examples, it is difficult to obtain the area of the color point as an unconditional solution due to the calculations since a number of (eight) colors are handled. Therefore, the following method has been proposed. Namely, in case of printing, since the rentinal points of different colors overlap, the color points of eight colors finely appear and it is therefore impossible to obtain the unconditional solution to derive three retinal point areas which can realize the color which should be reproduced. On the other hand, if the number of colors which are used for reproduction is limited to (R, G, B), (Y, M, C), K, and W, the calculations can be easily solved. A parenthesis ( ) denotes that one of the colors which are included therein is selected. However, different coefficients have to be used for six kinds of different combinations of primary colors used, namely, for six regions for color reproduction.
These six regions are represented by chromaticity coordinates of color reproduced; they are as shown in FIG. 1. In the chart, coordinates x and y are so-called variables indicating the chromaticity, and total seven points consisting of standard chromaticity Y, M and C of DIN, and R, G, B, and white point which are caused by them are represented as PY, PM, PC, PR, PG, PB, and PW, respectively.
The calculating expression to obtain the color point area in each region is the similar linear expression with three unknowns as that of the masking processing in the photograph and is of the same type of equation as a form. A different point is that the color densities of three layers are used as the variables in the photograph, while color stimulus specifications of three colors (simply, chrominance signals of three colors at the light intensity) are used in this case. Also in this case, different calculations have to be performed in six regions.
On the other hand, in case of executing a plurality of different calculation processings in accordance with the color which should be reproduced, two methods are considered:
(i) the chromaticity is examined and it is discriminated to see which region the chromaticity belongs to, then the correct processing is selected and the calculation is performed; PA0 (ii) look-up tables for the inputs and outputs are prepared in the whole chromaticity range and they are referred to. However, the method (i) needs the additional calculation for discrimination, so that this delays processing. The method (ii) requires a large storage device and in the case where the chromaticity of color point or three stimulus specifications which are used for color reproduction, namely, when the condition for color reproduction is changed, the content of the storage device has to be all replaced.